/Engine/Source/Editor/BehaviorTreeEditor/Classes/BehaviorTreeGraphNode.h
Add right click menu to create subnodes: Services
void AddContextMenuActionsServices ( class [UToolMenu](API\Developer\ToolMenus\UToolMenu) * Menu, const [FName](API\Runtime\Core\UObject\FName) SectionName, class [UGraphNodeContextMenuContext](API\Runtime\Engine\EdGraph\UGraphNodeContextMenuContext) * Context ) const